You may remember my post from some time ago about wanting to create a hybrid analog/digital system for FM, basically "HD Radio
" based on open standards. Well, I am delighted to say that 1 month and a BladeRF later, I have done it. Using DREAM
to modulate the digital signals, and Stereo Tool to generate the FM composite, You get something like this.
Now, the filters need to be tuned, and I think the filter of a typical FM radio is 200khz correct? That's what the design is based on.
Anyway, what does this offer? Well, like HD Radio, it allows you to simulcast your analog station in superior digital quallty. For instance, my digital setup has 3 "channels" spread over the two digital sidebands. The leftmost features a signle program, a simulcast of the analog feed - without any processing of course - at 64kbps. The second sideband carries a 40kbps secondary music program, and a 23kbps news/talk feed.
A digital signal being received. (This is the left sideband)
Those bandwidths seem low, but with the Opus audio codec
, they are optimal. 64kbps sounds somewhere between CD and DVD audio, whist 40kbps sounds like a fantastic FM signal with perfect stereo separation and no processing. The 23 kbps is in mono and sounds like a 5khz AM signal with perfect reception. All of this and the analog signal is still there. Complete with the usual Stereo and RDS.
I have also worked on another method which places a signal 20khz digital signal inside the FM envelope where the SCA signals would be. (Kind of like FMPlus). Though, a receiver for that would be absurdly complicated.
Tests so far have been fantastic, and I will be releasing the flowgraph, and documentation on how to get it all working by this weekend or early next week.
Any questions or comments, feel free to send them my way. :p